Stratigraphy, lithology, structure, and metamorphism of Permian, Triassic, and Tertiary rocks between the Mararoa River and Mount Snowdon, western Southland, New Zealand

Abstract

An area in western Southland underlain by Permian, Triassic, and Tertiary rocks is mapped and described geologically. New stratigraphic, lithologic, and petrographic data are presented, and several new mapping units are proposed. These latter include Upukerora Formation, a predominantly coarse-grained, volcanogenic sedimentary unit which occurs at the base of the Bryneira Group; Key Summit Volcanic Sandstone a characteristically massive green rock which underlies large portions of Bryneira Group outcrop north of the present area ; Snowdon Formation, a resistant conglomeratic and tuffaceous unit belonging to the Murihiku Supergroup; and the Annick and Waiau groups, Tertiary sediments of the Te Anau Basin. Several previously defined units are also mapped, described and in some cases, revised: Red Mountain, Livingstone, Howden, Anneal, Tapara, Winton, and Countess. Bryneira and Murihiku rocks comprise sparsely fossiliferous, richly volcanogenic geosynclinal strata which accumulated adjacent to an island arc. Red Mountain-Livingstone rocks are interpreted to be ancient oceanic crust and upper mantle upon which Bryneira sediments were deposited, the latter possibly filling in a frontal arc basin or inactive oceanic trench. The Snowdon Formation appears to overlie, possibly with mild unconformity, Bryneira strata, and was probably deposited in a frontal arc basin (arc-trench gap). Structurally, the area comprises the southern end of the Key Summit Regional Syncline. Red Mountain-Livingstone-Bryneira rocks lie entirely within the nearvertical to overturned east limb of the regional syncline; Red Mountain-Livingstone rocks comprise a tectonic melange throughout most of the area mapped. Snowdon Formation rocks lie near, or within, the axial portion of the syncline A newly recognised strike-slip fault, the Mararoa Fault, truncates the southern extent of Bryneira outcrop. This fault strikes northeasterly and has dextral offset probably exceeding 2–3 km. The Hollyford and Upukerora fault systems are shown to extend south from the Eglinton Valley area (Grindley 1958) into the present area. Lower-grade metamorphism has extensively altered mineralogy and microscopic texture of Permian and Triassic rocks. In addition some Tertiary strata of the Te Anau Basin contain authigenic zeolites. Authigenic hydrous Ca-Al-silicate minerals within Permian and Triassic rocks include lawsonite, laumontite, prehnite, pumpellyite and epidote. Areas within and transitional to five metamorphic zones are mapped on the basis of these minerals, and the zones are correlated with mineral facies of Coombs (1968). The mineral lawsonite is especially abundant and is taken to indicate moderately high pressure-low temperature conditions of recrystallisation. Metamorphism probably occurred during deep in-folding and tectonic pinching of the western margin of the New Zealand Geosyncline during the late Jurassic-early Cretaceous Rangitata Orogeny.